Pipelines are made of many different materials, including ductile iron, cast iron, welded steel, plastic, asbestos cement and concrete. They carry many different materials, such as water, waste, oil, gas, or chemical products. Many pipelines are buried underground, and are exposed to exterior corrosion and abrasion from the earth, gravel, sand or like material in which they are buried or from corrosive or abrasive substances within such material. Other pipelines suffer corrosion because of electrical charges, or become abraded and/or corroded by the material carried in the pipeline. Corrosion can cause pitting in metal pipelines, and breaking of reinforcing wires in concrete pipelines which have wire reinforcements within their walls. Pipelines can also be damaged or weakened by action caused by humans, as for example by heavy equipment passing over a buried pipeline.
In use, therefore, the walls of pipelines tend to become thinner and/or weaker over time. This can lead to rupture of the pipeline. It is therefore useful to determine those parts of the pipeline which are suffering from weakness or which have thinned, so that corrective action can be taken before the pipeline ruptures in a thinned or weakened location.
Current commercial methods of assessing wall thickness and/or strength are not very practical or cost-effective. It is possible to excavate a portion of a pipeline and make a small hole to get a sample (called a “coupon”) of the pipeline wall. This permits a direct measurement of the wall thickness and condition where the sample is taken. However, things which could cause thinning or weakening of a pipeline wall do not necessarily occur uniformly along the length of the pipeline. Therefore, a coupon reveals the situation at the precise location where it is taken, but does not necessarily give useful information about other locations along the pipeline.
It is also known to take direct ultrasonic measurements of pipeline wall thickness where a pipeline is exposed (i.e. not buried in the ground or otherwise inaccessible.) This is cheaper than taking coupons, but it is limited to exposed locations, and does not necessarily give useful information about other locations along the pipeline.
Acoustic measurement has also been used, as described for example in Hunaidi U.S. Pat. No. 6,591,032 and Hunaidi et al U.S. Pat. No. 7,328,618. By this method, the propagation velocity of a low frequency disturbance such as a release of fluid from a pressurized pipe or ambient pipe noise is determined by measuring the times at which the wave caused by this disturbance passes two fixed points which are spaced from each other by a known distance, for example two hydrants in a water line. The propagation velocity so determined is compared with a theoretically calculated velocity. The result is a figure which is said to represent the average thinning of the pipeline walls over the fixed distance. The average figure does not determine any specific locations between the fixed points where extreme thinning has occurred, or even reveal whether locations having greater-than-average thinning exist.
In some pipelines (particularly in the oil and gas industry), pipeline pigs are used to perform cleaning and data-gathering functions. Such pigs can be fitted with devices, such as ultrasonic measuring devices or magnetic flux measuring devices, which can determine wall thickness. However, the practical use of an instrumented pipeline pig is limited to pipelines which have pigging stations and which do not have sharp bends.
In some very large diameter pipelines, it has been possible to drain the pipeline and have personnel equipped with ultrasonic or similar devices take direct measurements at selected locations by walking through the pipeline to these locations. The draining of the pipeline takes it out of service, so this method is not suited to frequent checking of wall thickness.
It would be very useful to be able to determine where along the length of a pipeline the walls had become impaired, either by becoming thinner and/or weaker. It would be even more useful to be able to measure a quantitative value which represents relative impairment at locations along the pipeline. This information would permit appropriate maintenance or replacement to be undertaken of only those pipeline sections where the thinning or weakening had progressed to an undue extent.